linux-kernel - [PATCH 3.16 057/134] PCI: Ignore write combining when mapping I/O port space

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Message-ID: <lsq.1503062000.418077912@decadent.org.uk>
Date:   Fri, 18 Aug 2017 14:13:20 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Bjorn Helgaas" <bhelgaas@...gle.com>
Subject: [PATCH 3.16 057/134] PCI: Ignore write combining when mapping I/O
 port space

3.16.47-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Bjorn Helgaas <bhelgaas@...gle.com>

commit 3a92c319c44a7bcee9f48dff9d97d001943b54c6 upstream.

PCI exposes files like /proc/bus/pci/00/00.0 in procfs.  These files
support operations like this:

  ioctl(fd, PCIIOC_MMAP_IS_IO);           # request I/O port space
  ioctl(fd, PCIIOC_WRITE_COMBINE, 1);     # request write-combining
  mmap(fd, ...)

Write combining is useful on PCI memory space, but I don't think it makes
sense on PCI I/O port space.

We *could* change proc_bus_pci_ioctl() to make it impossible to set
mmap_state == pci_mmap_io and write_combine at the same time, but that
would break the following sequence, which is currently legal:

  mmap(fd, ...)                           # default is I/O, non-combining
  ioctl(fd, PCIIOC_WRITE_COMBINE, 1);     # request write-combining
  ioctl(fd, PCIIOC_MMAP_IS_MEM);          # request memory space
  mmap(fd, ...)                           # get write-combining mapping

Ignore the write-combining flag when mapping I/O port space.

This patch should have no functional effect, based on this analysis of all
implementations of pci_mmap_page_range():

  - ia64 mips parisc sh unicore32 x86 do not support mapping of I/O port
    space at all.

  - arm cris microblaze mn10300 sparc xtensa support mapping of I/O port
    space, but ignore the write_combine argument to pci_mmap_page_range().

  - powerpc supports mapping of I/O port space and uses write_combine, and
    it disables write combining for I/O port space in
    __pci_mmap_set_pgprot().

This patch makes it possible to remove __pci_mmap_set_pgprot() from
powerpc, which simplifies that path.

Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/pci/proc.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

--- a/drivers/pci/proc.c
+++ b/drivers/pci/proc.c
@@ -231,7 +231,7 @@ static int proc_bus_pci_mmap(struct file
 {
 	struct pci_dev *dev = PDE_DATA(file_inode(file));
 	struct pci_filp_private *fpriv = file->private_data;
-	int i, ret;
+	int i, ret, write_combine;
 
 	if (!capable(CAP_SYS_RAWIO))
 		return -EPERM;
@@ -245,9 +245,12 @@ static int proc_bus_pci_mmap(struct file
 	if (i >= PCI_ROM_RESOURCE)
 		return -ENODEV;
 
+	if (fpriv->mmap_state == pci_mmap_mem)
+		write_combine = fpriv->write_combine;
+	else
+		write_combine = 0;
 	ret = pci_mmap_page_range(dev, vma,
-				  fpriv->mmap_state,
-				  fpriv->write_combine);
+				  fpriv->mmap_state, write_combine);
 	if (ret < 0)
 		return ret;